Hybrid nanoporous pigment - cellulose nanomaterial composites for printed energy storage applications

We report pilot-scale manufactured low-cost pigment-cellulose nanofibril (PCNF) composite and its application as a separator-substrate in printed graphene and carbon nanotube supercapacitors. The developed low-cost nanoporous composites utilize novel raw materials based on micro- and nanoscale cellulose and inorganic pigment. In this study we varied composition of kaolin pigments and cellulose micro- or nanofibrils (CMF /CNF) fibrillated with different methods: grinding, fluidization and enzymatic. Totally four different CMF and CNF grades were tested in composites. We concluded that from unbleached softwood kraft pulp ground lignocellulose nanofibrils (LCNF), in mass portion of 20%, produced stiff and flexible film structures together with kaolin pigments (80%) and simultaneously had a good dewatering profile. Really smooth and even surfaces can be also manufactured from TEMPO oxidized cellulose nanofibrils (TCNF) and kaolin, but due to TCNF high fibrillation degree and relative surface area the dewatering of composites was a challenge. Overall, higher kaolin amounts (80%) improved dimensional stability of composites compared to lower amounts (50%) and the drying shrinkage during film forming was controllable. The forming parameters like pre-drying of composites, using of polyelectrolytes and activation method of cellulose micro or nanofibrils have a significant effect on the film properties like surface formation and smoothness. It was also demonstrated that in addition to the smoothness, the porosity is the most relevant property for the substrate. A semi-industrial scale pilot line was successfully used to produce smooth, flexible and nanoporous composites and their performance was tested in a double functional separator-substrate element in supercapacitors. Further, the nanostructural carbon films printed on the composite worked simultaneously as high surface area active electrodes and current collectors. Environmentally friendly materials used in low-cost supercapacitors have a potential to be used in flexible, wearable and disposable low-end products.